Adjustable piston pipettes comprise an upper and lower retainer of the piston movement, the location of at least one of said retainers can be changed. Generally the adjustment is realized by means of a screw-and-nut joint, for instance so that the retainer is provided with a screw, around which there is arranged a nut that does not rotate in relation to the housing but moves along guides along with the piston. The pitch of the screw threading defines the pace of the adjustment. Typically there are used 8–20 revolutions in the adjustment, depending on the size of the volume range. Usually pipettes of the described type also are provided with a calibration system whereby the basic position of the other retainer is set so that the dispensed liquid volume corresponds to the displaid volume as accurately as possible. Generally such pipettes also have a so-called secondary motion function. In said secondary motion, the desired liquid volume is sucked in the pipette by shifting the piston from the basic position to a desired upper position. When removing the liquid, the piston is depressed to a discharge position, somewhat below the basic position. Now liquid is removed from the pipette as completely as possible. In manually operated pipettes, the secondary motion function is realized so that they include a primary spring, against the force of which the piston is depressed from the upper position to the basic position, and a stronger secondary spring, against the force of which the piston is depressed to underneath the basic position.
A pipette of the described type is illustrated for instance in the publication FI 57543.
A general weakness of adjustable pipettes is their slowness in adjusting the volume. A screw-based adjusting operation can be speeded up by increasing the pitch of the threading. This, on the other hand, makes it more difficult to accurately set the desired volume, when already a slight turn changes the volume remarkably. The adjusting process becomes easier, if a high-pitched adjusting is realized in steps (see for example CH 679015). In that case, however, the close intermediate volumes are lost.